1. Field of the Invention
The present invention generally relates to a semiconductor integrated circuit, a protection circuit and a battery pack to protect a secondary battery by controlling an on/off status of a discharge control switch and a charge control switch.
2. Description of the Related Art
In recent years, an electronic device and the like driven by utilizing a battery pack that uses a secondary battery are widely used. The secondary battery is charged by allowing the battery pack to be mounted in a battery charger. The secondary battery may be charged by connecting an AC adapter and the like to the electronic device in which the battery pack is mounted.
Control of a charge and a discharge of the secondary battery is performed by a protection circuit that has a function of protecting the secondary battery from an overcharge and an over discharge, and a battery monitoring function of performing management of a remaining battery level and the like.
In such a protection circuit, for example, as a method of returning to a normal condition from a condition of detecting an over discharge (which is hereinafter called an over discharge return), two of a battery charger connecting return and a voltage return are known. The battery charger connecting return is a method of returning to the normal condition from the over discharge condition by detecting a connection of a battery charger to a battery pack. The voltage return is a method of returning to the normal condition from the over discharge by detecting a battery voltage of the secondary battery being charged up to an over discharge return voltage or more.
FIG. 1 is a diagram showing an example of a conventional protection circuit that adopts the battery charger connecting return. A protection circuit 10 shown in FIG. 1 includes a charge and discharge control IC (Integrated Circuit) 20, a switching transistor M1, a switching transistor M2, a resistor R1, a resistor R2, a B+ terminal, a B− terminal, a P+ terminal, and a P− terminal. Moreover, the charge and discharge control IC 20 includes a comparator 21, a standard voltage Vref, a logic circuit 22, a switch SW1, and a pull-up resistor R3.
A secondary battery B1 is connected between the B+ terminal and the B− terminal, and a battery charger CH1 or a load R7 is connected between the P+ terminal and the P− terminal. When the charge and discharge control IC 20 detects an overcharge from a battery voltage of the secondary battery B1, the logic circuit 22 outputs a control signal to turn the transistor M2 off from a terminal OV and stops charging.
If a load is connected between the P+ terminal and the P− terminal and the battery voltage of the secondary battery B1 becomes equal to or lower than an over discharge detection voltage, the charge and discharge control IC 20 detects an over discharge of the secondary battery B1. When the charge and discharge control IC 20 detects the over discharge, the logic circuit 21 outputs a control signal to turn the transistor M1 off from a terminal DCHG and stops the discharge.
A description is given about the over discharge return in the charge and discharge control IC 20. When the charge and discharge control IC 20 detects an over discharge, the logic circuit 22 turns the switch SW1 on, and an electric potential of a V− terminal is pulled up to a VDD potential by the load R7 and the pull-up resistor R3. If the load R7 connected between the P+ terminal and the P− terminal is removed and a battery charger CH1 is connected between the P+ terminal and the P− terminal, the electric potential of the V− terminal becomes a VSS potential that is not more than the reference voltage Vref. The charge and the discharge control IC 20 detects the electric potential of the V− terminal becoming not more than the reference voltage Vref, and the over discharge condition returns to a normal condition.
For example, Japanese Patent Application Laid-Open Publication No. 2010-124640 discloses a return from an over discharge condition. Also, Japanese Patent Application Laid-Open Publication No. 2002-034163 discloses a return from an over current condition.
In the conventional protection circuit 10, if the load is connected between the P+ terminal and the P− terminal, the battery voltage input into the protection circuit 10 is detected by monitoring a voltage drop caused by a discharge current flowing through the load R7. However, the battery voltage is estimated to be decreased by an amount of additional voltage drop caused by an internal impedance of the secondary battery B1 compared to the actual voltage. If an over discharge detection is performed in this condition, the over discharge detection is detected without using a battery capacity corresponding to the amount of additional voltage drop caused by the impedance of the secondary battery B1. The unavailable battery capacity becomes greater as the discharge current becomes greater.
Moreover, for example, in the conventional protection circuit, if a method of returning to the normal condition from the over discharge condition is the voltage return, and a hysteresis of an over discharge detection voltage and an over discharge return voltage is small, an oscillation that repeats the over discharge detection and the over discharge return may occur. This oscillation tends to occur easily when the discharge current is large. Furthermore, if the hysteresis is made large, a problem similar to the battery charger connecting return may occur.